A Structural, Bonding, and Properties Study of the Ordered Rock Salt Structures Li₂MO₃ (M = Ru, Ir, Pt)

The ordered rock salt structures of Li₂MO₃ have become of interest in recent years due to their potential applications as lithium ion battery and as pH sensing materials. Included in this class are the structures of Li₂IrO₃ and Li₂PtO₃ that can be derived from the well known Li-ion battery cathode material, LiCoO₂, via ordering of Li⁺ and Ir⁴⁺/Pt⁴⁺. The additional cation ordering lowers the symmetry from rhombohedral (R-3m) to monoclinic (C2/m). Unlike Li₂RuO₃ no evidence for further distortion of the structure driven by formation of metal–metal bonds. Thermal analysis studies coupled with both ex-situ and in-situ X-ray diffraction measurements show that these compounds are stable up to temperatures approaching 1375 K in O₂, N₂, and air, but decompose at much lower temperatures in forming gas (5% H₂:95% N₂) due to reduction of the transition metal to its elemental form. Li₂IrO₃ undergoes a slightly more complicated decomposition in reducing atmospheres, which appears to involve loss of oxygen prior to collapse of the layered Li₂IrO₃ structure. Electrical measurements, UV–visible reflectance spectroscopy and electronic band structure calculations show that Li₂IrO₃ is metallic, while Li₂PtO₃ is a semiconductor, with a band gap of 2.3 eV.

Further analysis of faulting in Li₂IrO₃ and Li₂PtO₃ systems is presented, the degree of which is dependent on the heating temperatures used. Faulting analysis was carried out using Rietveld refinements as well as refinements using the software FAULTS based on powder X-ray diffraction (PXRD). Additionally, PXRD and selected area electron diffraction simulations were carried out using DIFFaX that are subsequently compared to experimental results. FAULTS analysis yields alpha₁₂ values, transition probabilities, of 66.2%, 87.3%, and 94.7% for LIO-750, LIO-900, and LIO-1050, respectively, while they are 74.8%, 85.8%, and 96.8% for LPO-750, LPO-900, and LPO-1050, respectively.

Density Functional Theory(DFT) calculations and lattice energy calculations were also performed with manganese, tin, zirconium and lead being placed in the Li₂MnO₃, Li₂SnO₃, and Li₂ZrO₃ structure types. These results indicate that ionically, the Li₂ZrO₃ structure is the most stable. Upon considering covalent interactions the layered structures of Li₂MnO₃ and Li₂SnO₃ become more stable for both Sn and Mn, but not for Zr. Thus these results are consistent with expectations. Additionally, DFT calculations substantiate the occurrence of Ru‐Ru dimerization in Li₂RuO₃, while also predicting the as yet unseen metal‐metal bonding in Li₂IrO₃ and the theoretical Li₂OsO₃ and Li₂ReO₃ structures.